This invention relates to apparatus for transmitting and/or receiving microwave radiation comprising means for generating signals at microwave frequency, an antenna having individual elements arranged at progressively higher levels, and means for feeding the signals to the individual elements in such a way that the amplitudes and phases of the signals at the individual elements cause the antenna to have a gain which is relatively low at negative elevation angles (i.e., angles below the horizontal); which rises steeply to a maximum at a low positive elevation angle; and falls (preferably relatively slowly and at a progressively decreasing rate) towards higher elevation values.
The need for a gain distribution in the vertical plane as described above is apparent from FIGS. 1 and 2. FIG. 1 is a schematic illustration which assumes an antenna to be located at the origin. This antenna forms part of a radar system at an airport to detect aircraft within a given horizontal range (d) and below a maximum height (h). It is not required to detect aircraft at elevation angles higher than 35.degree.. Thus, the shaded area of FIG. 1 indicates the region, in vertical plane, that it is designed to survey. This requirement for a radar to survey an area like that shown shaded on FIG. 1 is typical for radars required to monitor the activities of aircraft in the region of an airport and gives rise to the need for a radar antenna having a gain which varies with elevation in a manner as shown by a solid line in FIG. 2. It is not detrimental if the gain is higher than the required value (i.e., above the solid line of FIG. 2) at positive elevation angles. It is however a disadvantage for the gain to be above a specified level (ideally zero) at a negative elevation angle since, if it were, a substantial amount of radiation would be transmitted onto the ground and cause the radar to respond to signals transmitted and/or received indirectly by reflection off the ground.
An approximation to the gain distribution in the vertical plane, as illustrated by the solid line of FIG. 2, has generally been achieved in the past using a method called Woodward Synthesis to calculate appropriate phase and amplitude values to be applied to individual elements of an antenna. Using the Woodward method one might typically design the antenna so that the amplitude and phase distributions are as shown in dot-dash lines in FIGS. 3A and 3B: assuming that the antenna elements are located in a vertical plane. It should be explained here that it is not essential that the antenna elements be located in a vertical plane. They could be located in a sloping plane as will be described later.
Referring now to FIG. 3A, and in particular to the dot-dash line therein, it is notable that, using Woodward Synthesis, the amplitude increases at an increasing rate in lower and upper base regions of the antenna, reaches and falls from a peak in a central region, and drops at a decreasing rate towards the top of the antenna.
Referring now to FIG. 3B it will be seen that, again using the Woodward technique, indicated by the dot-dash line, the phase lag, relative to a reference, is also generally symmetrical about the centre of the antenna. In a central region it rises relatively rapidly, whilst in the top and base regions it rises relatively slowly. The curve thus has two distinct bends indicated at 1 and 2 in the central region where the second derivative of phase with respect to height has peaks.
The amplitude and phase distributions, e.g., as shown in FIGS. 3A and 3B, calculated according to the Woodward method, typically give a gain distribution somewhat as shown by the dot-dash line in FIG. 2. From FIG. 2 it will be noted that this gain distribution features a high side lobe 3 at a negative elevation angle. It also features one or more troughs 4 which fall below a CSC.sup.2 part 5 of the ideal curve.